Delegates, can't get my head around them

Question

Hey, I'm looking for useful resources about Delegates. I understand that the delegate sits in the background and receives messages when certain things happen - e.g. a table cell is selected, or data from a connection over the web is retrieved.

What I'd like to know in particular is how to use delegates with multiple objects. As far as I know, specifying the same delegate for an object (e.g. table cell) would cause the same events to be called for both the cells at the same time. Is there anything equivalent to instantiating a delegate for a particular object?

Thanks in advance!

Answer 1

In Cocoa, objects almost always identify themselves when calling a delegate method. For example, UITableView passes itself as the first parameter of the delegate message when calling it:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath

If you wanted the same delegate to handle multiple UITableViews, then you just need a some conditional on the tableView object passed to the method:

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    if (tableView == self.myFirstTableView) {
        // do stuff
    } else if (tableView == self.mySecondtableView) {
        // do other stuff
    }
}

}

If you don't want to compare the object pointers directly, you can always use the tag property to uniquely identify your views.

Answer 2

Usually, if you have a delegate method that might have to receive messages from many different objects, you simply have the calling object pass itself to the delegate in the message (method call).

For example, if you wanted a delegate method to extract the text from a tableviewcell's label, the method definition would look something like:

-(void) extractTextFromLabelOfTableCell:(UITableViewCell *) theCallingCell{
...
NSString *extractedText=theCallingCell.textLabel.text;
}

You would call the method from a tableviewcell thusly:

[delegate extractTextFromLabelOfTableCell:self];

Each instance of the tableviewcell would send itself to the delegate and the delegate would extract that instance's text. In this way, a single delegate object could handle an arbitrarily large number of cells.